The present invention relates to an adhesive tape having a superior flame resistance and a substrate for the adhesive tape.
A conventional adhesive tape comprising a substrate for an adhesive tape (hereinafter to be also referred to simply as a substrate), which is made from polyvinyl chloride (hereinafter to be abbreviated as PVC), is superior in mechanical properties (particularly flexibility and stretchability), flame resistance, resistance to thermal deformation, and electrical insulation property. Combined with its beneficial economical aspect, the above-mentioned tape has been widely used as an insulating tape for electric equipment used in the fields of vehicles, such as automobile, train, bus and the like, aircraft, ship, house, plant and the like. Particularly, an adhesive tape to be wound around a wire harness to be used for electric wires in automobile and the like, a coil for household electric appliances, an electric wire and the like, is required to show high flame resistance (oxygen index of not less than 25%) and high resistance to thermal deformation. To meet these requirements, an adhesive tape using PVC as a substrate has been widely used.
Given the recent rise of consciousness toward environmental problems, however, there is a movement to reduce use of PVC and replace it with a material causing less environmental burden. This is because PVC is suspected of generating dioxin and toxic gas, such as chlorine gas, upon incineration. To replace PVC, the use of a polyolefin resin as a substrate has been considered, because it less likely generates dioxin or toxic gas, such as chlorine gas, upon incineration. However, polyolefin resins easily burn as compared to PVC and require addition of a flame retardant. For example, U.S. Pat. No. 4,067,847 proposes addition of an inorganic flame retardant, such as magnesium hydroxide and aluminum hydroxide, which places only a small burden on the environment.
In consideration of the flexibility and stretchability necessary for an adhesive tape, polypropylene, ethylene-propylene copolymer, high density polyethylene and the like, having a relatively high melting point, are not suitable as the polyolefin resin used along with a flame retardant. Instead, a polyolefin resin having a low melting point needs to be used, such as low density polyethylene, linear low density polyethylene, ultra-low density polyethylene, middle density polyethylene, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer and the like. These polyolefin resins having a low melting point are flexible and stretch easily, but are susceptible to thermal deformation.
As a method for improving resistance to thermal deformation of a resin film, there have been known a method comprising exposing the film to ionizing radiation, a method comprising previous addition of a crosslinking agent, such as organic peroxide (e.g., dicumylperoxide), to a resin and vapor heating of a film after forming to provide a crosslinking structure in the film, and the like. These methods also increase the number of production steps and production cost to the level that makes their practical use unfeasible.
A different method comprises the use of a mechanically blended polymer mixture of polypropylene, ethylene-propylene copolymer, high density polyethylene and the like, having a relatively high melting point, and an elastomer consisting of ethylene copolymer, such as EPM (ethylene-propylene copolymer rubber), EBR (ethylene-butene rubber), EPDM (ethylene-propylene-diene copolymer rubber) and the like. When a flame retardant is added to this polymer mixture, a molded product thereof becomes very stiff and lacks flexibility at room temperature, making stretchability strikingly low and resistance to thermal deformation insufficient.
It is therefore an object of the present invention to provide an adhesive tape free of generation of dioxin and toxic gas upon incineration, which has high levels of resistance to thermal deformation and flame resistance, as well as a substrate to be used for this adhesive tape.
It is also an object of the present invention to provide an adhesive tape having a high level of stretchability that is beneficial in winding a tape, in addition to high levels of the resistance to thermal deformation and flame resistance, as well as a substrate to be used for this adhesive tape.
According to the present invention, a thermoplastic resin having a carbonyl oxygen atom in the molecular skeleton and a polymer alloy containing an ethylene component and a propylene component, in combination, are used as an olefin polymer, and a flame retardant is added to give a substrate. As a result, a substrate for an adhesive tape, which shows high levels of resistance to thermal deformation and stretchability, as well as a high level of flame resistance, can be obtained.
Accordingly, the present invention provides an adhesive tape comprising a substrate and an adhesive layer formed on at least one side of the substrate, wherein the substrate comprises an olefin polymer and a flame retardant, but substantially no halogen atom, and the adhesive tape shows a thermal deformation at 100xc2x0 C. of not more than 65%.
In a preferable embodiment, the adhesive tape shows the following properties. It shows an elongation at break of not less than 150% at a tension speed of 300 mm/min. It shows a dynamic storage modulus (Exe2x80x2) at 80xc2x0 C. of not less than 25 MPa and a dynamic storage modulus (Exe2x80x2) at 120xc2x0 C. of not less than 10 MPa.
The present invention provides a substrate for an adhesive tape, which contains an olefin polymer and a flame retardant but contains substantially no halogen atom, wherein the olefin polymer contains the following Component A and Component B:
Component A: a thermoplastic resin having a carbonyl oxygen atom in the molecular skeleton
Component B: a polymer alloy containing an ethylene component and a propylene component
In a preferable embodiment, the Component B shows the following properties. It shows a dynamic storage modulus (Exe2x80x2) at 80xc2x0 C. of not less than 40 MPa and less than 180 MPa, and a dynamic storage modulus (Exe2x80x2) at 120xc2x0 C. of not less than 12 MPa and less than 70 MPa. It shows a dynamic storage modulus (Exe2x80x2) at 23xc2x0 C. of not less than 200 MPa and less than 400 MPa. Component A is an ethylene copolymer having a melting point of not more than 120xc2x0 C., which is obtained by polymerizing a vinyl ester compound and/or xcex1,xcex2-unsaturated carboxylic acid or a derivative thereof, or a metal salt of the ethylene copolymer. Component A and Component B are mixed at a weight ratio (A:B) of 1:9-8:2. The flame retardant is added in an amount of 20-200 parts by weight per 100 parts by weight of an olefin polymer. The flame retardant is a metal hydroxide.
In another preferable embodiment, the substrate has a dynamic storage modulus (Exe2x80x2) at 80xc2x0 C. of not less than 25 MPa, and a dynamic storage modulus (Exe2x80x2) at 120xc2x0 C. of not less than 10 MPa. The substrate is not crosslinked during or after its formation.